1. Technical Field
This invention relates to hermetically sealed packages and, more particularly, to dewars containing infrared detectors.
2. Discussion
Some sensors, particularly mercury-cadmiumtelluride infrared detectors, are most sensitive when operating at approximately 77.degree.K. These detectors are typically used in conjunction with an evacuated dewar in which the detector is placed. The evacuation of the dewar is used to remove gasses which would otherwise occupy the region surrounding the detector so that the potential heat loss through convection and conduction during operation is minimized, as well as to eliminate the formation of liquid onto the detector. The detector is generally mounted onto the tip of a coldfinger which is in communication with a cryoengine assembly. During operation the cryoengine serves to expand a fluid such as helium in the coldfinger which, in turn, adsorbs thermal energy causing the detector to be cooled.
While the traditional evacuated dewar has generally operated satisfactorily, it does have some drawbacks. For example, the choice of materials that are used to fabricate the dewar is somewhat limited and expensive because it is necessary to choose materials having special characteristics such as low diffusivity, low outgassing and other properties. Furthermore, implementing the necessary closure techniques required to create the vacuum inside the dewar is often costly and it is sometimes difficult to ensure that the vacuum is maintained over a long period of time.
U.S. Pat. No. 4,719,353 discloses a non-evacuated dewar in which polymeric foam is disposed between the expander or coldfinger and the housing. While the above document discloses a dewar which has its advantages, it also has its own set of shortcomings and can be further improved.